An undifferentiated cell that undergoes meiosis to produce an egg (ovum).
Oogenesis
Gametogenesis in the ovary, leading to the formation of mature ova.
Operator
A site on DNA that interacts with a repressor protein, regulating transcription of an operon.
Operon
A segment of DNA consisting of a promoter, operator, and structural genes. The structural genes code for products of a specific biochemical pathway; their transcription is regulated by a repressor protein.
Organ
A somewhat independent body part that performs a special function.
Organelle
A specialized structure of a cell, such as a mitochondrion, Golgi complex, lysosome, endoplasmic reticulum, ribosome, centriole, chloroplast, cilium, or flagellum.
Osmosis
The diffusion of pure solvent across a membrane in response to a concentration gradient, usually from a solution of lesser to one of greater solute concentration.
Ovary
The female gonad: either of the paired female sexual glands in which oocytes are formed.
Oviduct (Fallopian tube)
A passage through which ova leave the maternal body or pass to an organ communicating with the exterior of the body.
Ovulation
The discharge of an ovum from the ovary.
Ovum
The female reproductive cell or gamete; egg.
Oxidation
A reaction in which the atoms in an element lose electrons and the valence of the element is correspondingly increased.
